The present invention relates to a method for using a phase-change material for preserving animal and human body parts, biological cultures and the like. More particularly, the present invention relates to a new method for using the phase change materials of the type disclosed on U.S. Pat. No. 4,856,294, incorporated by reference herein, such that extremely temperature sensitive specimens are adequately protected via multi-phase cooling using latent heat.
Prior to the use of multi-phase change materials described in the aforementioned U.S. patent for personal cooling systems, it was common to use cold water pumped through pipes in a vest. Among the several disadvantages of using cold water was that the water was considerably colder than the desired skin temperature. This, in turn, caused discomfort for the wearer of the vest. By using a multi-phase-change material, the inventors maintained the temperature cooling action for a living human being within a practical and comfortable range. Other types of phase change material garments are disclosed in U.S. Pat. Nos. 5,111,668 and 5,113,666.
We have now recognized that other things, namely extremely sensitive temperature sensitive specimens such as medical specimens, bacteria, biological cultures and body parts which often have to be shipped long distances, can be ruined from excessive cold or heat. Thus, ice cannot be used in this application because it produces an undesirably low temperature in addition to being undesirably heavy. The conventional approach has been to use chilled water cooled to about 50.degree. F. along with insulation. This is the so-called single phase cooling approach which has limited cooling ability because only "sensible" cooling (that is, mCp.DELTA.T) is used. It also is undesirably heavy due to the weight of water.
We have now discovered that the phase-change materials disclosed in above-referenced U.S. Pat. No. 4,856,294 provide a substantially improved technique for maintaining living tissue, medical specimens and cultures at the appropriate temperature.
A major advantage of using such material in accordance with the present invention is that it does not allow container contents to become too hot or too cold.
Another advantage of the present invention is that the new use of particular phase-change materials allows exploitation of latent heat occurring from solid-liquid phase change. For example, sensible heating from 50.degree. F. to 70.degree. F. is only 20 BTU/lb with single phase cooling whereas latent heat at 65.degree. is 101 BTU/lb or about five times the cooling capacity available via single phase cooling.
Still a further advantage of using a phase-change material is that the phase change material will also provide active thermal control in contrast to single phase sensible heating. The thermal control is available due to the nature of phase change materials, because the phase change material melts or solidifies at a constant temperature. Therefore, if cooling is required, (e.g., the ambient temperature in the container is warmer than the surroundings), the phase change material will melt, thereby absorbing heat from the surroundings and providing cooling. Likewise, if there is heating required (e.g., the ambient temperature is colder than the surroundings), the phase change material will solidify, thereby rejecting heat to the surroundings and providing heating.
The new use which we have recognized for such phase change materials is thus ideal for the shipment of perishable, temperature sensitive items such as human and animal organs, blood plasma, body parts, biological cultures, and bacteria which have to be shipped long distances (air shipments) and are extremely temperature sensitive.